As a conventional image forming apparatus for printing on both sides of paper, an image forming apparatus disclosed in Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 11-341231 (JP 11-341231; published on Dec. 10, 1999) is known, for example. As shown in FIG. 1 of the publication, the two-side image forming apparatus is provided with a transcription drum for transcribing, on a sheet, an image that is read by an image reading section 1a. When transcribing the image on the surface of the sheet P, a sheet P sent from a sheet stock section 5 to a main transferring path (an example of a first sheet-transferring path) 6 passes by a transcribing section provided in vicinity of the transcribing drum 19, whereby the image is transcribed on a surface of the sheet. The thus transcribed image is fixed on the surface of the sheet by a fixing roller 23. The sheet on which the image is fixed, is held by switch-back rollers 10, which rotate forwardly (normal rotation), and is transferred so that an rear end of the sheet is held by the switch-back rollers 10.
Then, the switch-back roller 10 rotates backwardly (reverse rotation) so as to enter the sheet P into a sub-transferring path (an example of a second sheet-transferring path). Then, the sheet P is transferred by a transferring roller 11 provided in the sub-transferring path 8, so as to enter the sheet P into the main transferring path 9 again. Turn-over of the sheet P is carried out by entering the sheet P into the sub-transferring sheet 8 by using the reverse rotation of the switch-back roller 10, and then reentering the sheet P into the main transferring path 6 from the sub-transferring path 8. The turn-over causes a reverse surface of the sheet p to face the transcribing drum 19. Therefore, it is possible to transcribe the image on the reverse surface of the sheet P passing along the main transferring path 6.
After the image is transcribed onto both the sides of the sheet P, the sheet P is delivered out into a deliver-out tray 12 by normal rotation of the switch-back roller 10.
According to the conventional two-side image forming apparatus described in JP 11-341231 having the above-mentioned arrangement, two-side printing is carried out by continuous transfer of sheets thereby attaining a shorter process time. Further, the use of the sub-transferring path, which requires only small area, leads to miniaturization of the apparatus.
In the conventional two-side image forming apparatus, however, only one sheet is transferred through both the main transferring path 6 and the sub-transferring path 8 during the two-side printing of one sheet. Thus, it is not possible to carry out the two-side printing by printing plural sheets parallel. In this point, the conventional two-side image forming apparatus has a problem in printing efficiency.
It is conceived to improve the printing efficiency of the two-side printing by transferring plural sheets parallel during the printing. Japanese Publication of Unexamined Patent Application, Tokukai, No. 2001-171884 (published on Jun. 26, 2001) discloses an image forming apparatus in which a sheet (following sheet) is entered into a turning-over path while a sheet (followed sheet) entered before the sheet is passing along the turning-over path. In this method, the following sheet enters into the turning-over path after the followed sheet reaches leading means for leading a turned-over sheet to a backward transferring path.
However, this method cannot improve the printing efficiency sufficiently and fails to respond to a need to improve efficiency in complete sheet transfer including reversing of sheet for printing both surfaces of the sheet. Thus, there is a demand for a two-side image forming apparatus for printing plural sheets with more efficient sheet transfer of the plural sheets without causing sheet-jamming in the sheet transfer.